Despite their prevalence and public health significance, major unanswered questions exist regarding the mechanisms involved in vulnerability to bipolar spectrum disorders (BSDs). Social and circadian (24-h) rhythm models of BSD risk hypothesize that mood episodes may be a result of circadian rhythm abnormalities caused by life events that disrupt social rhythms (e.g., bedtimes, mealtimes) that normally entrain circadian rhythms. Recent research also provides strong support for a Behavioral Approach System (BAS)/reward hypersensitivity theory of BSDs. Although research on these distinctive theoretical models has proceeded in parallel, our recent work demonstrates influences of reward sensitivity on social rhythm disruption and mood symptoms, leading us to a novel integration of reward sensitivity and social/circadian disruption in this proposal. Our overarching goal is to use an innovative biobehavioral high-risk design to examine bidirectional influences of reward sensitivity and social and circadian rhythm disruption as risk factors for BSD mood symptoms/ episodes. We will prospectively follow 210 participants (Ps) drawn in part from an existing sample of High and Moderate BAS/reward sensitive Ps at an age of risk for BSDs. We will compare High BAS Ps with a BSD diagnosis, High BAS Ps who have not yet exhibited but are at risk for BSD, and Moderate BAS Ps with no BSD to determine whether reward hypersensitivity influences social and circadian rhythm disruption following the occurrence of reward-relevant events to predict manic and depressive symptoms/episodes. This 3-group design will allow us to test whether reward hypersensitivity and its relationship with social and circadian rhythm disruption are pre-existing vulnerabilities or consequences of BSD, or both. Social rhythm regularity and reward sensitivity will be assessed at baseline. Reward-relevant life events, social rhythm disruption from these events, and mood symptoms/episodes will be assessed prospectively for up to 4 years. Ps will complete a 4-wk ecological momentary assessment (EMA) study including 1-wk baseline, 2-wk high BAS/reward activation, and 1-wk reward-outcome periods to assess bidirectional influences between social and circadian rhythm disruption and reward sensitivity and their synergistic effects on BSD symptoms. In the EMA study, Ps will complete daily measures of life events, social rhythms, and sleep diaries, continuous measures of sleep/wake (actigraphy) and circadian rhythms (skin temperature), and repeated within-day measures of mood, symptoms, and reward motivation. Dim light melatonin onset will be assessed 3 times to determine circadian melatonin phase changes. This research program will provide valuable insights into the mechanisms underlying vulnerability to BSDs and contribute to the development of targeted intervention and prevention strategies.